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## why wa?求助啊！！

Posted by 415943706 at 2009-02-22 15:30:06 on Problem 2079
```//我的算法是先算凸包，再枚举凸包上的点，一开始一直TLE，现在加了两个建枝一直WA
//求求高手帮帮小弟，不慎感激

#include <stdio.h>
#include <math.h>
#include <stdlib.h>

#define MAXSIZE 50001

typedef struct
{
double x;
double y;
}POINT;

POINT result[MAXSIZE];							//保存凸包上的点
POINT a[MAXSIZE];
int n,top;

double Distance(POINT p1,POINT p2)			//两点间的距离
{
return sqrt((p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y));
}

double Multiply(POINT p1,POINT p2,POINT p3) //叉积
{
return ((p2.x-p1.x)*(p3.y-p1.y)-(p2.y-p1.y)*(p3.x-p1.x));
}

int Compare(const void *p1,const void *p2)
{
POINT *p3,*p4;
double m;
p3=(POINT *)p1;
p4=(POINT *)p2;
m=Multiply(a[0],*p3,*p4) ;
if(m<0) return 1;
else if(m==0&&(Distance(a[0],*p3)<Distance(a[0],*p4)))
return 1;
else return -1;
}
void Tubao()
{
int i;
result[0].x=a[0].x;
result[0].y=a[0].y;
result[1].x=a[1].x;
result[1].y=a[1].y;
result[2].x=a[2].x;
result[2].y=a[2].y;
top=2;
for(i=3;i<=n;i++)
{
while(Multiply(result[top-1],result[top],a[i])<=0)
top--;
result[top+1].x=a[i].x;
result[top+1].y=a[i].y;
top++;
}
}

int main()
{
int i,j,k,p;
double px,py,temp;
while(scanf("%d",&n)!=EOF)
{
if(n<0) break;
for(i=0;i<n;i++)
scanf("%lf%lf",&a[i].x,&a[i].y);

py=-1;
for(i=0;i<n;i++)
{
if(py==-1 || a[i].y<py)
{
px=a[i].x;
py=a[i].y;
p=i;
}
else if(a[i].y==py && a[i].x<px)
{
px=a[i].x;
py=a[i].y;
p=i;
}
}
//swap(a[0],a[p])
temp=a[0].x;
a[0].x=a[p].x;
a[p].x=temp;
temp=a[0].y;
a[0].y=a[p].y;
a[p].y=temp;

qsort(&a[1],n-1,sizeof(double)*2,Compare);
a[n].x=a[0].x;
a[n].y=a[0].y;
Tubao();

double max_area = 0; //最大三角形面积
double area=0;
for(i=0;i<top-2;i++)
{
j=i+1,k=j+1;
double pre=-1;
while(j<top-1)
{
area=Multiply(result[i],result[j],result[k])/2.0;
while(k<top)
{
temp=Multiply(result[i],result[j],result[k])/2.0;;
if(temp>=area)
{
k++;
area=temp;
}
else break;
}
if(area<pre) break;
pre=area;
if(max_area<area) max_area=area;
j++;
}
}
printf("%.2lf\n",max_area);

}
return 0;
}
```

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